What is Tradeable in Hearthstone?
Hearthstone has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Tradeable keyword.
Play the card normally or swap it with a random one from your deck by paying 1 mana.
Tradeable is a keyword added in United in Stormwind. Cards with Tradeable can be dragged to your deck which will pay 1 mana and draw a different card from your deck (a huge mechanical departure from what we usually get in Hearthstone).
Many cards with Tradeable are reliant on a few factors. Because they have Tradeable though, they aren't completely dead cards in matchups/situations where the effect can't be utilized well as you can pay 1 mana to draw a different card giving you some extra flexibility in this manner.
Keywords that Work Great with Tradeable
By its nature, there are no mechanics that work with Tradeable itself as it is a very different from anything else in the game. Some Tradeable cards though are synergistic with keywords in more indirect ways. An example of this is Overdraft which synergizes with both Spell Damage and Overload, not from its Tradeable effect but its nature as a spell that deals damage. These don't directly work with Tradeable, but do allow the card to be traded away if you draw it without any Overload or Spell Damage activators in your hand.
Playing Against Tradeable in Hearthstone
Tradeable cards will reveal themselves to the other player when traded. This information can sometimes be useful. For example, if you see your opponent trade away a Rustrot Viper, you know that they won't have it in their hand anymore and thus it is safer to play a weapon in your hand (as tech cards are often only run at one copy unless the card is extremely important for the metagame). In a similar vein, if you see an opponent Trade away an Overdraft and then you notice they Overload a lot on a particular turn, then one may take this as a sign that they drawn Overdraft again (whether it be the same traded copy or a different one).
If you suspect your opponent is holding a Tradeable card in their hand that you want to get rid of, this can be done with hand disruption effects although this can backfire in the event that your opponent isn't holding the card or that you simply hit the wrong card.
Example Hearthstone Tradeable Cards
Rustrot Viper is a tech card with Tradeable that destroys the opponent's weapon when played. Although a situational effect, it can be traded away for a different card in the event that you draw it in a matchup where the effect isn't relevant.
Fire Sale is a board clear with Tradeable. If you don't need a board clear at the moment, you may be tempted to hold on the card anyway as you might need it relatively soon, but you can also trade it to try to draw a card that's more useful in the current situation.
Overdraft is a useful Shaman card that will unlock all your Overloaded Mana Crystals and deal equivalent damage to a minion. While very powerful, you may end up drawing it without any Overload cards in your hand. You however can Trade it away until you get some.
Impatient Shopkeep is a much more simple example of the mechanic. Though it doesn't use the mechanic in any clever way as some of the other cards with it do, it can still be a good idea to trade away a 3/3 with Rush if it's not particularly useful at the current moment.